(1) Field of the Invention
The invention relates to a process for fabricating a photomask, and more particularly, to a process for fabricating a phase-shifting photomask.
(2) Description of the Prior Art
As photolithography advances to 0.25 microns and below, new technologies are required to increase resolution of the imaging lens. One new technology is the use of phase-shifting photomasks. Phase shift masks take advantage of the interference effect in a coherent or partially coherent imaging system to reduce the spatial frequency of a given object, to enhance its edge contrast, or both.
FIG. 1A illustrates a top view of a typical binary mask 10 comprising a chromium portion 12 and a glass portion 14. FIG. 1B illustrates the electric field behind the mask of FIG. 1A. FIG. 1C illustrates the slope of intensity of the mask in FIG. 1A. FIG. 2A illustrates a top view of a phase-shifting mask 11. Chromium portion glass portion 14 are shown. The phase-shifting portion causes a 180xc2x0 phase shift. The electric field behind this mask is shown in FIG. 2B. FIG. 2C shows the intensity slope of the mask in FIG. 2A. The higher slope of intensity in FIG. 2C than in FIG. 1C indicates higher contrast and therefore, better resolution in the phase-shifting mask as compared to a binary mask.
The partial transmittance of the light waves through the phase-shifting portion 16 allows for phase shifted light to be produced. While phase shift masks can enhance resolution and increase depth of field of the lithography process, they are on the order of five times as expensive to produce as binary masks. It is desired to find a process for making a phase shift mask that is both simple and inexpensive.
U.S. Pat. No. 5,747,196 to Chao et al discloses a process for forming a phase shift mask in which both a chromium layer and a light-transmissive layer are formed over glass. The two films are anisotropically etched to form a pattern where they are not covered by a resist. The resist is partially eroded away and then the chromium is etched away where not covered by the eroded resist to form the PSM. This process requires the use of a light-transmissive layer over the glass to form the phase-shifting portion which adds cost to the process. U.S. Pat. No. 5,290,647 to Miyazaki et al teaches a PSM process including using polysilicon as an etching mask. U.S. Pat. No. 5,418,095 to Vasudev and U.S. Pat. No. 5,536,603 to Tsuchiya et al disclose other processes for making phase shift masks.
Accordingly, it is a principal object of the present invention to provide an effective and very manufacturable process for fabricating a phase-shifting photomask.
Another object of the present invention is to provide a process of fabricating a phase-shifting photomask using a dry etch process to etch the glass of the mask a step high where the step height is equal to {fraction (1/4+L )} wavelength of the exposure tool. 
Yet another object of the present invention is to provide a process of fabricating a phase-shifting photomask using an ashing process to pattern the light-blocking film.
A further object is to provide a process of fabricating a phase-shifting photomask using a dry etch process to etch the glass of the mask a step high where the step height is equal to {fraction (1/4+L )} wavelength of the exposure tool and using an ashing process to pattern the light-blocking film. 
A still further object is to provide a process for fabricating a phase-shifting photomask in which an additional light-transmissive film is not required to form the phase-shifting portion of the photomask.
In accordance with the objects of this invention a new process for fabricating a phase-shifting photomask is achieved. A photomask blank is provided comprising a chromium layer overlying a substrate and a resist layer overlying said chromium layer. The resist layer of the photomask blank is exposed to electron-beam energy and developed away whereby a first resist pattern remains. The chromium layer not covered by the first resist pattern is etched away and, simultaneously, the substrate not covered by the first resist pattern is etched into to a depth in namometers of {fraction (1/4+L )} the wavelength of the exposure tool whereby a substrate step is formed underlying the first resist pattern. Thereafter, a portion of the first resist pattern is ashed away to leave a second resist pattern smaller than the first resist pattern and exposing portions of the chromium layer underlying the first resist pattern. The exposed portions of the chromium layer not covered by the second resist pattern are etched away whereby portions of the underlying substrate step are exposed. The second resist pattern is removed to complete fabrication of the phase-shifting photomask.